1. Field of the Invention
The invention concerns techniques to assess the electromagnetic effects associated with the deployment of antennas having transverse or tilted axes within a borehole. More particularly, the invention relates to techniques for detecting or monitoring the borehole effect encountered in subsurface measurements.
2. Description of Related Art
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically employ logging instruments or “sondes” equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy interacts with the surrounding formation to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile or log of selected physical parameters of the formation properties is obtained.
Electromagnetic (EM) logging techniques known in the art include “wireline” logging and logging-while-drilling (LWD). Wireline logging entails lowering the instrument into the borehole at the end of an armored electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD entails attaching the instrument disposed in a drill collar to a drilling assembly while a borehole is being drilled through earth formations.
Conventional wireline and LWD instruments are implemented with antennas that are operable as sources and/or sensors. In wireline applications, the antennas are typically enclosed by a housing constructed of a tough plastic material composed of a laminated fiberglass material impregnated with epoxy resin. In LWD applications, the antennas are generally mounted on a metallic support to withstand the hostile environment encountered during drilling. Conventional logging instruments are also being constructed of thermoplastic materials that provide a non-conductive structure for mounting the antennas.
In both wireline and LWD applications, the antennas are mounted on the support member and axially spaced from each other in the direction of the borehole. These antennas are generally coils of the cylindrical solenoid type and are comprised of one or more turns of insulated conductor wire that is wound around the support. U.S. Pat. Nos. 4,873,488 and 5,235,285 (both assigned to the present assignee), for example, describe instruments equipped with antennas disposed along a central metallic support. In operation, the transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (“mud”) and into the formation. The signals detected at the receiver antenna are usually expressed as a complex number (phasor voltage) and reflect interaction with the mud and the formation.
One EM logging technique investigates subsurface formations by obtaining electrical resistivity or conductivity logs by “focused” measurements. U.S. Pat. No. 3,305,771 describes a focusing technique using an instrument equipped with toroidal coils. U.S. Pat. Nos. 5,235,285 and 5,339,037 (both assigned to the present assignee) describe metallic instruments adapted with a toroidal coil and electrode system for obtaining resistivity measurements while drilling. The measurement techniques described in the '285 and '037 patents entail inducing a current that travels in a path including the conductive support body and the formation.
U.S. Pat. Nos. 5,396,175, 3,388,325, 3,329,889, and 3,760,260 (all assigned to the present assignee) describe instruments equipped with electrode and/or coil configurations for obtaining subsurface measurements. U.S. Pat. No. 4,538,109 (assigned to the present assignee) describes a logging technique aimed at correcting or canceling the effects of spurious EM components on downhole measurement signals.
A coil or loop-type antenna carrying a current can be represented as a magnetic dipole having a magnetic moment of strength proportional to the current and the effective area of the coil. The direction of the magnetic moment vector is perpendicular to the plane of the coil. In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the instrument. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole.
An emerging technique in the field of well logging is the use of instruments incorporating antennas having tilted or transverse coils, i.e., where the coil's axis is not parallel to the support axis. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). The aim of these TMD configurations is to provide EM measurements with directed sensitivity and sensitivity to the anisotropic resistivity properties of the formation.
Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 4,319,191, 4,360,777, 5,508,616, 5,115,198, 5,757,191, 5,781,436, 6,044,325, 6,147,496, 6,163,155, and 6,181,138.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is much more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current the borehole axis. When it is eccentered in a direction parallel to the direction of the transverse component of the magnetic moment, the symmetry of the situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the transverse component of the magnetic moment, axial currents are induced in the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD receivers, they can cause undesired signals that are many times larger than would appear in a homogeneous formation without a borehole.
U.S. Pat. No. 5,041,975 (assigned to the present assignee) describes a technique for processing signal data from downhole measurements in an effort to correct for borehole effects. U.S. Pat. No. 5,058,077 describes a technique for processing downhole sensor data in an effort to compensate for the effect of off-center tool rotation while drilling. However, neither of these patents relates to the monitoring or assessment of these borehole effects to determine other subsurface or instrument parameters.
Thus there remains a need for techniques to detect or monitor the borehole effect when using well logging instruments implemented with TMDs. A need also remains for improved techniques to determine the orientation or position of such instruments within a borehole.